1. Field of the Invention
The present invention relates generally to acoustic structures that are used to attenuate noise. More particularly, the present invention is directed to providing acoustic septum material for use in acoustic structures to provide a relatively low non-linearity factor (NLF) for noise attenuation.
2. Description of Related Art
It is widely recognized that the best way of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures (acoustic treatments) to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic treatments are typically incorporated in the engine inlet, nacelle and exhaust structures. These acoustic treatments include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, such as engine nacelles, acoustic materials are added to the honeycomb structure so that the honeycomb cells are acoustically closed at the end located away from the engine and covered with a porous covering at the end located closest to the engine. The closing of the honeycomb cells with acoustic material in this manner creates an acoustic resonator that provides attenuation, dampening or suppression of the noise. Acoustic septums are also usually located within the interior of the honeycomb cells in order to provide the resonator with additional noise attenuation properties.
The materials used to form acoustic septums and other acoustic structures typically include numerous holes that are an essential part of the acoustic properties of the material. The holes are typically drilled mechanically or by using a laser. Once formed, the cross-sectional area of the holes remains constant. The inability to actively change size and/or shape of septum holes in response to changes in noise pressure and gas velocity presents certain problems with respect to noise sources, such as jet engines, where the velocity of air or gas emitted from the engine varies with engine speed and location.
Nonlinearity factor (NLF) is a standard measure of a septums ability to attenuate noise over a range of flow velocities. NLF is typically determined by measuring the flow resistance of the septum at a low flow rate (e.g. 20 cm/second) and a high flow rate (e.g. 200 cm/second). The ratio of the low flow rate resistance to the high flow rate resistance is the NLF. It is desirable that the NLF be as close to 1 as possible. An NLF of 1 means that the flow resistance and sound dampening capability of the septum material remains constant as the flow velocity of air or gas through the septum increases.
A popular septum material is fabric made from woven monofilaments of certain polymers, such as polyetheretherketone (PEEK). These types of woven fabric septums tend to have relatively low NLF's which are typically below 2. However, such woven monofilament PEEK septums are relatively expensive.
The less expensive drilled septum materials tend to have NLF's on the order of about 4 and more. It would be desirable to provide relatively inexpensive septums made from the same septum material as drilled septums, but where the openings are formed and oriented such that the NLF of the septum is comparable to woven monofilament septum material.